The present invention is related to photolithography and photomasks used in the fabrication of semiconductor microchips and more particularly to preventing or reducing the damage caused by the discharge of electrostatic fields between metallic patterns on the reticle plate or photomask.
Electrostatic damage (ESD) is a well-known and commonly understood phenomenon in microelectronics. This phenomenon seriously impacts photomasks used in the fabrication of micro circuitry and results in unexpected and undiscovered defects on the reticle or photomask which can then be used to print defective semiconductor chips causing substantial yield and photolithographic manufacturing problems in semiconductor fabrication lines.
ESD events or discharges can occur on reticles when a build-up charge on one portion of the pattern discharges or causes a current to flow from one portion of the pattern to any surrounding point that is not at the same potential. If the potential difference is sufficient to cause breakdown of the intervening insulating medium, the metallic pattern such as chromium or molybdenum may melt or evaporate and then the melted metallic vapors or material may be re-deposited in the spaces between the intended pattern lines or shapes. This will generate undesired defects that will then be printed on the silicon wafer. As will be understood by those skilled in the art, if the defective photomask is not detected at the time the ESD event occurs, the defective photomask may be used to print thousands and thousands of microchips on various wafers. It is then not until later testing and quality control that the defect would be discovered. By that time, millions of dollars may have been lost due to the defective photomask.
It is not the electrostatic charging alone which causes damage to the reticles, it is the actual discharge between two isolated portions of the pattern (usually metallic) having different potentials. Various techniques already exist for the dissipative discharging of the charge on the mask and include ionized air, conductive glows, conductive shoes, floor mats, hand bands, and the use of electrically conductive materials of construction of the reticle carriers in transportation pods. Unfortunately, the charge still builds up in a significant number of cases, and it is the discharge which causes the damage.
Therefore, it is an object of the present invention to provide a low cost photomask or reticle and method of manufacturing such a reticle or photomask that provides substantial protection against ESD damage.
It is another object of the present invention to provide a method of manufacturing reticles and photomasks and to produce a resulting reticle and photomask which does not require the costly procedures of preventing an electrostatic buildup on a reticle or photomask.
These and other objects are achieved by the present invention which comprises a photomask and a method of manufacturing a photomask which is protected against ESD or electrostatic damage. According to the invention, there is included a substrate such as fused quartz or silica which is transparent to the wavelength of light used for printing circuits or microchips on a wafer. The substrate includes a front face and a back face with a pattern permanently applied or adhered to the front face. The pattern is opaque to the wavelength of light used for printing and is typically a metal such as chromium and molybdenum suicide. A conductive film which is also transparent to light having a wavelength used for printing is deposited at least over those portions of the front face of the substrate not covered by the opaque pattern. For example, according to one embodiment, the conductive transparent film covers the entire front face including a previously deposited pattern. However, according to another embodiment, the conductive transparent film is deposited before the pattern is deposited and covers the entire front face. The pattern is then deposited over the combination substrate and conductive transparent film. The xe2x80x9ctransparentxe2x80x9d conductive film is selected from a group of materials consisting of ITO (Indium Tin Oxide), Palladium, Platinum, Gold and conductive polymers depending on the wavelength of the light chosen for printing. In addition, the thickness of the deposited layer of film will also be dependent upon the wavelength of the light used for printing.